Advances in polymerization and catalysts have resulted in the capability to produce many new polymers having improved physical and chemical properties useful in a wide variety of superior products and applications. With the development of new catalysts, the choice of polymerization-type (solution, slurry, high pressure or gas phase) for producing a particular polymer has been greatly expanded. Also, advances in polymerization technology have provided more efficient, highly productive and economically enhanced processes. Regardless of these technological advances in the polyolefin industry, common problems, as well as new challenges still exist. For example, the stable operation of a gas phase process at high production rates utilizing dew point increasing components remains a challenge, which can particularly be dependent on the polymer being produced, the catalyst system employed, and the particular dew point increasing component employed.
Unstable fluidization, agglomeration, fouling, sheeting and/or static generation in a continuous gas phase process, in, for example, the fluidized bed, heat exchangers, distributor plates, and probes, can lead to the ineffective operation of various reactor systems. In a typical continuous gas phase process, upward flowing cycle gas fluidizes a bed of resin particles. The cycle gas is removed from the top of the reaction vessel as a recycle stream, compressed and passed through a cooler, and returned back into the bottom of the reaction vessel. This recycle system is employed for many reasons, including the removal of heat generated in the process by the polymerization reaction. An interruption, diversion, or blockage of the flow of cycle gas through any part of the fluidized bed can result in significant operating problems.
It is well known that stable operation of fluidized bed reactors used in the production of polymers requires the avoidance of conditions that lead to sticky polymer or fusion of resin particles in the fluidized bed. Sticky, or cohesive polymer causes a range of problems in the gas phase reactor systems. For example, sticky polymer can reduce the quality of fluidization that occurs within the reactor, and can reduce the degree of internal mixing below the minimum levels required to disperse the catalyst and maintain stable temperature control. The most common result of excessive resin stickiness is the formation of small rounded agglomerates that accumulate above the plate and disturb fluidization. In addition, stickiness of the polymer can lead to the deposition of polymer product on the walls of the reactor expanded section, which often leads to the formation of dome sheets (solid masses of polymer material deposited on the walls of the “dome”, or expanded section of the reactor). In many cases, these dome sheets are large and massive, containing as much as 1000 kg of agglomerated polymer. These dome sheets eventually fall from the dome and become lodged on the distributor plate, where they interfere with fluidization. In some cases, the dome sheets block the product discharge port, and force a reactor shutdown for cleaning. In more extreme cases, a large dome sheet can disrupt fluidization in a localized region above the plate and lead to formation of a chunk. For these reasons it is desirable to have means of preventing excessive stickiness of the polymer product.
Polymer stickiness is thought to be a function of several process and product variables within the reactor. The relevant process variables include the reaction temperature and the concentrations (or partial pressures) of condensable components such as 1-butene and isopentane in the reactor gas phase. In general, stickiness of the polymer is promoted by higher reaction temperature and higher concentrations of condensable materials. Important product properties include the resin density, molecular weight (or melt index), and the molecular weight distribution (MWD). In general, stickiness of the polymer is promoted by lower resin density, lower molecular weight (higher melt index), and broader molecular weight distribution (Mw/Mn=MWD).
It is also well known that polymer leaving the gas phase reactor contains significant quantities of dissolved gases, including monomers, comonomers, and dew point increasing components. Furthermore, a quantity of reactor cycle gas is also entrained with the polymer leaving the reaction system. These dissolved and entrained gases are separated from the polymer in a polymer purging system. The entrained gases, dissolved gases, and other gases exiting the reaction system are recovered in vent recovery systems using methods of compression, chilling, and condensing.
Fluidized-bed reactors used to produce polyethylene resin are normally operated at a relatively high reaction temperature. For example, in the production of a typical low density film resin (0.917 g/cc density, 1 dg/min melt index) produced with metallocene or Ziegler-Natta catalyst, the reaction temperature is typically operated at 85° C. A relatively high reactor temperature provides for a relatively high temperature differential over the cooling water temperature (which typically operates at 25 to 35° C.). This, in conventional practice, is thought to provide for maximum heat removal capability for maximum production rates.
It would be desirable to have a polymer production process that is free of polymer agglomeration or stickiness. It would also be desirable to have a process that allows higher concentrations of condensable materials and/or higher dew point temperatures in the reactors for higher production rates. It is even further desirable to operate with a higher level of condensable components while improving the recovery of these condensable components from the purge vessel and other reactor vent streams.
Our findings indicate that when low molecular weight dew point condensing components are included in the reactor cycle gas, the previously determined maximum dew point of the cycle gas relative to the bed temperature had been unnecessarily limited due to concerns regarding polymer stickiness. We found that it is possible to operate with a dew point that is closer to the bed temperature than previously thought by increasing the amount of low molecular weight dew point increasing component and actually increase maximum production rates, while avoiding problems of resin stickiness. We also found that the recovery of condensable components from the purge bin and reactor vent streams can be improved through the use of an enhanced recovery system.